In new study, ancient and modern evidence suggests limits to future global warming

April 20, 2006

DURHAM, N.C. - Instrumental readings made during the past century offer ample evidence that carbon dioxide and other "greenhouse gases" in the atmosphere are warming Earth's climate, a team led by Duke University scientists has reported. But by analyzing indirect evidence of temperature fluctuations over six previous centuries, the team also found that the magnitude of future global warming will likely fall well short of current highest predictions.

In making their deductions, the researchers ran some 1,000 computer simulations, covering 1,000 years, that took into account a range of modern and ancient climate records. Modern records are based on thermometer readings, while measurements derived from such sources as tree rings and ice cores served as markers of warm and cold spells over prior centuries.

The investigators evaluated the data using an "energy balance model" that they describe as a slimmed-down version of the heavy-duty computer models typically used to analyze climate trends. It is the model's streamlined nature that enabled the researchers to perform such large numbers of simulations over such a long period in such detail, they said.

The group used thousands of different versions of this model, each version varying in some of its properties, in order to determine which variants best matched actual observations. One key property that varied was what the researchers termed "sensitivity" - that is, how much the simulations' temperatures would change in response to increasing greenhouse gas levels.

"What I can say very confidently is that the present-day sensitivity is not zero, meaning that there is a positive, warming response to greenhouse gases," said climate analyst Gabriele Hegerl, an associate research professor at Duke's Nicholas School of the Environment and Earth Sciences. "Our work also substantially reduces the probability of very high climate sensitivities."

Hegerl is lead author of the study, published April 20, 2006, in the journal Nature. Her co-authors are Thomas Crowley, Duke's Nicholas Professor of Earth Systems Science; William Hyde, a former Nicholas School research scientist now at the University of Toronto; and David Frame, a researcher at the University of Oxford.

Their work was supported by the National Oceanic and Atmospheric Administration, the U.S. Department of Energy and the National Science Foundation.

Many scientists expect that the level of carbon dioxide in the atmosphere will sometime this century reach double the levels that were present during preindustrial times. Because carbon dioxide traps outgoing heat energy similarly to the glass in a greenhouse, the additional human-created outputs of the gas - mostly from fossil-fuel burning - are expected to warm Earth's climate. The key question is: by how much?

The commonly accepted range for how much average global temperatures will rise in response to a doubling of atmospheric carbon dioxide is between 1.5 and 4.5 degrees centigrade, according to the researchers. But some observational studies, they noted, suggest the possibility that average temperatures might rise more than 9 degrees.

However, the new study - using "reconstructions" of Northern Hemisphere temperatures since the year 1270 - indicates a 90 percent probability that a doubling of carbon dioxide levels will result in temperature increases of between 1.5 and 6.2 degrees, the team reported.

In turn, the study showed a reduced likelihood that the actual maximum increase will exceed 4.5 degrees - "from 36 percent to 15 percent or less," the researchers said. A 4.5 degree increase is the highest maximum currently predicted by the international Intergovernmental Panel on Climate Change.

Hegerl said her group confined its study largely to the Northern Hemisphere because only there have scientists collected enough data to reconstruct temperature variations over the entire past millennium.

According to Hegerl, some studies claim that preindustrial temperatures fluctuated very little until the past century, and have risen sharply since.

"But our reconstruction supports a lot of variability in the past, as well as an upward trend in the 20th century," she said. And a record with plenty of ups and downs before the modern era "shows a climate reacting then and now to a variety of 'external forcing,'" she said.

The term "external forcing" refers to all those outside influences that can perturb the climate. Understanding how temperatures responded to such forcings in the premodern era - when the impact of carbon dioxide and other heat-trapping gases varied relatively little - helps scientists predict future forcings by greenhouse gases, Hegerl said.

"Looking back longer in time makes it possible to more confidently rule out responses that are very high or very low," she said.

The researchers consulted instrumental records of the various forcings that have occurred in modern times, with the aim of comparing those to actual recorded temperatures.

In order to reconstruct temperatures from the centuries before 1850, the team used various lines of indirect evidence. They looked, for example, at particulates trapped in ice cores as measures of past volcanic eruptions. Such eruptions eject clouds of particles high into the atmosphere. By reducing the amount of sunlight that can pass through the atmosphere, the particles tend to cool the climate for a time, Hegerl said.

They also consulted a number of tree ring studies that reveal hot and cold spells in ancient growth variations, as well as studies that can estimate temperatures as far back as the 1600s based on readings obtained from holes bored deep into the ground.

Although the researchers collected data spanning a full millennium, because of some technical limitations they actually simulated temperature variations over a roughly 700-year period beginning in 1270.

All in all, the researchers considered four different detailed reconstructions of past climates, including a new reconstruction done by Crowley and Hegerl, to deduce probable temperatures before reliable instruments were available.

According to Hegerl, past volcanic eruptions provided the strongest tie between past climate forcings and temperatures. "You can see downturns in temperature exactly where you see volcanic eruptions," she said.

Mammoths killed by abrupt climate change New research has revealed abrupt warming, that closely resembles the rapid man-made warming occurring today, has repeatedly played a key role in mass extinction events of large animals, the megafauna, in Earth's past.

New Ice Age may begin by 2030 The arrival of intense cold similar to the one raged during the "Little Ice Age", which froze the world during the XVII century and in the beginning of the XVIII century, is expected in the years 2030--2040.

Oceans slowed global temperature rise, scientists report A new study of ocean temperature measurements shows that in recent years, extra heat from greenhouse gases has been trapped in the subsurface waters of the Pacific and Indian oceans, thus accounting for the slowdown in the global surface temperature increase observed during the past decade, researchers say.

Major Midwest flood risk underestimated by as much as 5 feet, study finds As floodwaters surge along major rivers in the midwestern United States, a new study from Washington University in St. Louis suggests federal agencies are underestimating historic 100-year flood levels on these rivers by as much as five feet, a miscalculation that has serious implications for future flood risks, flood insurance and business development in an expanding floodplain.

Climate change won't reduce winter deaths In a study that contradicts the received wisdom on health impacts of climate change, scientists say that we shouldn't expect substantial reduction in winter deaths as a result of global warming.

Risk of major sea level rise in Northern Europe Global warming leads to the ice sheets on land melting and flowing into the sea, which consequently rises. New calculations by researchers from the Niels Bohr Institute show that the sea level in Northern Europe may rise more than previously thought.

Climate Change: Evidence and Causes is a jointly produced publication of The US National Academy of Sciences and The Royal Society. Written by a UK-US team of leading climate scientists and reviewed by climate scientists and others, the publication is intended as a brief, readable reference document for decision makers, policy makers, educators, and other individuals seeking authoritative information on the some of the questions that continue to be asked. Climate Change makes clear what is well-established and where understanding is still developing. It echoes and builds upon the long history of climate-related work from both national academies, as well as on the newest climate-change assessment from the United Nations' Intergovernmental Panel on Climate Change. It touches on current...

Tirelessly promoted by princes, presidents, actors and activists, "climate change" has become a dominant theme of global politics. But what's really going on as the "pause" in global warming prepares to enter its third decade? In this new anthology, leading scientists and commentators from the United States, the United Kingdom, Canada and Australia consider the climate from every angle - the science, the policy and the politics.

WINNER OF THE PULITZER PRIZEONE OF THE NEW YORK TIMES BOOK REVIEW'S 10 BEST BOOKS OF THE YEARA NEW YORK TIMES BESTSELLERA NATIONAL BOOK CRITICS CIRCLE AWARD FINALISTA major book about the future of the world, blending intellectual and natural history and field reporting into a powerful account of the mass extinction unfolding before our eyes

Over the last half-billion years, there have been Five mass extinctions, when the diversity of life on earth suddenly and dramatically contracted. Scientists around the world are currently monitoring the sixth extinction, predicted to be the most devastating extinction event since the asteroid impact that wiped out the dinosaurs. This time around, the cataclysm is us. In prose that is at once frank, entertaining, and deeply...

Global Warming-Alarmists, Skeptics & Deniers: A Geoscientist looks at the Science of Climate Change, brings a unique geological perspective to this politically charged issue, a perspective that has been ignored far too long. Written by a father-son team of geoscientist and attorney, it is the concise guide to the global warming controversy that has been long needed. As a university professor and research geologist for thirty years, Dr. Robinson knows that geological science is essential for placing the global warming controversy in proper prospective. One cannot hope to understand how humans might be causing climate change without an understanding of the magnitude and speed natural processed are capable of when it comes to climate change. Earth history is the only yardstick we have to...

"Merchants of Doubt should finally put to rest the question of whether the science of climate change is settled. It is, and we ignore this message at our peril."-Elizabeth Kolbert "Brilliantly reported andwritten with brutal clarity."-Huffington Post Now a powerful documentary from the acclaimed director of Food Inc., Merchants of Doubt was one of the most talked-about climate change books of recent years, for reasons easy to understand: It tells the controversial story of how a loose-knit group of high-level scientists and scientific advisers, with deep connections in politics and industry, ran effective campaigns to mislead the public and deny well-established scientific knowledge over four decades. The same individuals who claim the science of global warming is "not settled" have also...

The Great Global Warming Blunder unveils new evidence from major scientific findings that explode the conventional wisdom on climate change and reshape the global warming debate as we know it. Roy W. Spencer, a former senior NASA climatologist, reveals how climate researchers have mistaken cause and effect when analyzing cloud behavior and have been duped by Mother Nature into believing the Earthâ€™s climate system is far more sensitive to human activities and carbon dioxide than it really is.

In fact, Spencer presents astonishing new evidence that recent warming is not the fault of humans, but the result of chaotic, internal natural cycles that have been causing periods of warming and cooling for millennia. More carbon dioxide in the atmosphere is not necessarily to be feared; The...

Rachel Carson's epoch-creating Silent Spring marked the beginnings of the environmental movement in the 1960s, its 'First Wave' peaking at the 1972 Stockholm Conference. The invention of sustainable development by Barbara Ward, along with Rachel Carson the founder of the environmental movement, created an alliance of convenience between First World environmentalism and a Third World set on rapid industrialisation. The First Wave crashed in 1973 with the Yom Kippur War and decade-long energy crisis. Revived by a warming economy of the 1980s, environmentalism found a new, political champion in 1988: Margaret Thatcher. Four years later at the Rio Earth Summit, politics settled the science. One hundred and ninety-two nations agreed that mankind was causing global warming and carbon dioxide...

Award-winning science writer Seymour Simon gives you a full-color photographic introduction to the causes and effects of global warming and climate change. Earth's climate has always varied, but it is now changing more rapidly than at any other time in recent centuries. The climate is very complex, and many factors play important roles in determining how it changes. Why is the climate changing? Could Earth be getting warmer by itself? Are people doing things that make the climate warmer?Supports the Common Core State Standards

What is climate? Climate is commonly thought of as the expected weather conditions at a given location over time. People know when they go to New York City in winter, they should take a heavy coat. When they visit the Pacific Northwest, they should take an umbrella. Climate can be measured as many geographic scales - for example, cities, countries, or the entire globe - by such statistics as average temperatures, average number of rainy days, and the frequency of droughts. Climate change refers to changes in these statistics over years, decades, or even centuries. Â Enormous progress has been made in increasing our understanding of climate change and its causes, and a clearer picture of current and future impacts is emerging. Research is also shedding light on actions that might be taken...

â€śThe author makes it very clear that a changing climate is not unusual. It is in fact the norm -- although change is often so slow that humans cannot detect it directly. Â But historical geologists know how to display the evidence to the public. This may be the single most important contribution of this well-written and fact-filled book.â€ťâ€”Dr. S. Fred Singer, Chairman Science & Environmental Policy Project

â€śGlobal Warming: Alarmists, Skeptics and Deniers is a refreshing read on a topic of great societal importance; refreshing because, unlike many books published on this subject, the authors of this work evaluate key predictions and controversies of the global warming debate using logic and science.Â Most readers will appreciate the bookâ€™s arrangement.Â Each chapter...